The purpose of these studies is to establish a better understanding of energy metabolism in intact tissues. Toward this goal, this laboratory concentrates on the use of non-invasive and techniques to evaluate the biochemical function of the heart. These techniques include optical and nuclear magnetic resonance spectroscopy to monitor various aspects of tissue function. In the past year we have concentrated on establishing the cytosolic feedback mechanism operating between oxidative phosphorylation and mechanical work in the intact heart. The following major findings were made: 1) Using optical spectroscopy, we have shown that the oxygen tension of the heart, under control of blood flow autoregulation, is below the maximum concentration for oxidative phosphorylation. This data demonstrates that the heart actively maintains the oxygen tension below the saturation point of this process, indicating that tissue oxygen tension could play an active role in the limitation of energy metabolism. 2) Using a Ca++ sensitive dye in the intact heart, we have demonstrated that the mean intracellular Ca++ (Ca++i) concentration increases in proportion to the increase in cardiac afterload work and oxygen consumption. In paired experiments, the mitochondrial NADH concentration increases in proportion to Ca++i. This is consistent with the notion that Ca++i could regulate the rate of oxidative phosphorylation by modulating the NADH redox state via mitochondrial dehydrogenases. 3) The effects of ketone infusions on the metabolism of the heart in vivo was established demonstrating that the mitochondrial NADH redox state is a viable control point in the regulation of oxidative phosphorylation in vivo.